Here a clear distinction must be drawn between (1) the planned NATO deployment of American medium-range missiles due to begin this December, and (2) the specific Western proposal on the table at the American-Soviet Euromissile arms control talks in Geneva.

Mr. Kohl's conservative backers are rock firm in supporting the first and ambiguous on the second. The Social Democrats - with the luxury of possessing neither power nor the policy responsibility of power, are ambiguous to negative on both, but do not propose any clear alternatives.

In this election period, however, both political rivals are painting their adversaries' nuclear policy as more black and white than it really is. The Social Democrats are coming close to labeling the conservatives the ''missile party''; the conservatives charge the Social Democrats with heading toward neutralism and constituting a ''security risk.''

Kohl's public statements leave no doubt that he supports prompt implementation of NATO's planned deployments if there is no arms control agreement by December. Only the certainty that NATO will in fact deploy, argue Kohl and other NATO leaders, will bring enough pressure on Moscow to bargain away its present virtual monopoly of several hundred land-based Euromissiles against NATO's planned missiles.

Kohl's public statements about the American negotiating position of ''zero option'' are less clear, however. Under this proposal the Soviet Union would have to dismantle all of its already deployed 333 SS-20s (and 280 old SS-4s and SS-5s) for NATO to forego its planned 572 new medium-range missiles.

Kohl still calls this the best solution. But the West German chancellor also leaves the door open to compromise by specifying, in what is now the standard formula, that this is no take-it-or-leave-it proposition. In addition, the Kohl government, under the signature of Foreign Minister Hans-Dietrich Genscher, confidentially warned Washington at year-end of potential American-European strains if Washington is rigid in the Euromissile negotiations.

What the West German government would hope for from more American flexibility would be either an arms control agreement - or, barring Soviet willingness to compromise, winning European public support for new missiles once the United States had thus proved itself to be negotiating in good faith.

Here a comparison with French Socialist President Francois Mitterrand is instructive. Mitterrand, who unabashedly seeks deployment of new US missiles in Europe to counter the five-year buildup of Soviet SS-20s, has indicated his distaste for the US ''zero option.'' He wants at least some new US missiles on European, and especially West German soil in order to prevent the ''decoupling'' of European from American security.

Mitterrand also fears a lurch of the West German Social Democrats toward neutralism if the Social Democrats win the March election, as he all but said in a highly unusual speech in the West German Bundestag last month. He thus joins Brzezinski in this concern, as does West German Foreign Minister Genscher.

The Liberal Genscher, despite campaign pressure to distance himself from the Social Democratic coalition partners he dropped last fall, does not go as far as Brzezinski, however, in seeing neutralism as already prevailing among the Social Democrats; instead, he warns against trends in the Social Democratic Party that could lead to neutralism.

Chief among these, Genscher suggests, are the tendency toward ''equidistance'' between the two superpowers, enthusiasm for nuclear-free zones along the East-West German border - and dismissal of new NATO missiles as tools for the Reagan administration to wage nuclear war in Europe rather than a deterrent shield for Europe. Such inclinations, Genscher warned in a Feb. 11 interview in Die Zeit, could lead to Bonn's eventual dissociation from NATO and subjection to Soviet pressure.

Indeed, the Social Democratic position has long been that both reluctant superpowers must be prodded to serious arms control. The position of the Social Democratic left wing, as voiced most recently by the party's disarmament spokesman Egon Bahr in the party newspaper Vorwarts, has been that Washington needs this prodding rather more than Moscow. (This latter position tends to be shared by the US peace movement, though probably not by most American arms control advocates.) The left wing also clearly yearns to postpone NATO deployments if there is no arms control agreement by December.

The standing Social Democratic platform, on the other hand, still supports both parts of NATO's deploy-but-negotiate decision. And Social Democratic Chancellor candidate Hans-Jochen Vogel keeps saying only - rather -noncommit-tally - that he wants to do everything possible to make the planned NATO missile deployments ''superfluous.''

For his part, ex-Chancellor Helmut Schmidt of the Social Democrats' pragmatic wing has been saying remarkably little about the issue - except to deny the common assumption that he was the father of the NATO conclusion that it now needs intermediate-range nuclear weapons.

The Social Democrats are thus distancing themselves as much as possible from the new NATO missiles without actually revising their formal support for the NATO ''two-track'' (deploy-but-negotiate) decision. This avoids a lethal battle over the issue between the party's left and right wings. It also works to reabsorb antinuclear protesters back into mainstream politics.

Whether it will also work to push the Social Democrats toward neutralism once the March 6 election is over remains to be seen.